1. Field of the Invention
The present invention relates to phased array antennas. More particularly it relates to a system and method for monitoring the signal radiated by a phased array antenna which provides an indication of faults in components of the antenna and identification of the faulty components. The invention also provides an accurate and convenient method for calibrating a phased array antenna for initial use.
Phased array antennas are found in a variety of applications primarily because of their ability to produce a radiation pattern of specified characteristics which may be steered electronically to any desired angle within certain coverage limits. The phased array antenna application of particular interest herein is in the Scanning Beam Microwave Landing System, but it is to be understood that the invention may be used in conjunction with phased array antennas in other applications.
2. Description of the Prior Art
A linear phased array antenna used in a Scanning Beam Microwave Landing System is described in U.S. Pat. No. 3,999,182 issued Dec. 21, 1976 to A. W. Moeller et al. In this antenna a plurality of radiating elements are spaced equally along a linear axis. The elements of the array lying to the left and to the right of the center element are each fed r.f. energy through individual electronically variable phase shifters, each of which is coupled into right and left branching series feed lines through individual directional couplers. The center elements of the array and the right and left branch series feed lines are coupled to a common microwave source through a four-way directional coupler.
The number of radiating elements and associated phase shifters and couplers present in an array is dependent upon such design factors as the desired beam width and sidelobe levels of the radiation pattern of the array and the scan angle coverage of the array. The elevation antenna described in the referenced patent includes eighty-one directional couplers and provides a radiation pattern of 1.degree. beam width with a maximum sidelobe level of -27 dB. The scan angle coverage is from 0.degree. to +20.degree. in elevation.
Specific radiation pattern characteristics of the array require that each element coupler be designed to supply a precisely determined portion of power from the feed to the associated element and that each element be coupled into the feed with a particular insertion phase. The numerous components involved in a phased array antenna increase the number of sources of possible failure in the system. In compensation, however, a failure occurring in any one component does not result in total failure of the system but only results in a marginal degradation in beam quality. As the number of failed components increases, the beam quality decreases correspondingly until the deterioration exceeds a tolerable level and the system can no longer be safely used.
Monitoring systems have heretofore been used in conjunction with phased array antennas in the Microwave Landing Systems to warn of impending or actual system failure or to identify specific component failures, such as a diode failure in a digital phase shifter. One such monitor system comprises a receiver located at some distance from the antenna along a particular radial or elevation angle. The equivalent of a receiver located at a fixed point in the far field of the antenna is provided by an integral monitor usually comprising a slotted waveguide extending the length of the antenna array in close proximity thereto. Energy from the antenna array is coupled into the waveguide through the slots with the proper phases to produce a signal at the waveguide output corresponding to the signal which would be received at a distant point along a fixed radial from the array. The detected signal from the distant receiver or integral monitor provides data from which the beam main lobe and sidelobe signal strengths and pointing angles can be measured. Comparison of measured values of these quantities with stored values of similar quantities obtained during calibration of the array can reveal departure of the system performance below an acceptable level. Such a monitor is suitable for on-line executive use to alert operating personnel to the need to remove the system from service for maintenance. Such a monitor does not identify the system component or components at fault. Further measures must be taken to isolate and correct or compensate for the malfunction. These additional measures are taken while the array is out of service and involve determining the amplitude and phase of the monitor signal for each element of the array, element by element.
One disadvantage of such prior methods of fault identification and fault compensation of a phased array is the necessity to provide a highly efficient r.f. switch for each element of the array. Another disadvantage is that the test procedures cannot be conducted while the array is operating in service.
The present invention is in a method of processing data from a distant monitor receiver or from an integral monitor antenna which is capable of detecting and identifying non-time varying amplitude and phase faults as to each element of a phased array antenna during on-line operation of the array.
It is an object of the invention to provide a monitoring system for a phased array antenna capable of identifying special faults in the antenna during operation of the antenna in service.
It is another object of the invention to provide a monitoring system for a phased array antenna capable of determining phase insertion errors for each radiating element of the array.
Still another object of the invention is to provide a monitoring system for a phased array antenna capable of identifying particular elements of the array having faults in their associated r.f. feedlines, couplers or connectors.
It is a further object of the invention to provide a monitoring system for a phased array antenna capable of automatically calibrating the array to provide specific phase correction factors to compensate for phase shifter errors resulting from manufacturing tolerances.
Briefly, the invention comprises a method of sampling and processing data collected by a single monitor receiver located in the far field of a scanning phased array antenna, or by an equivalent integral monitor, which enables the antenna illumination function to be received. The beam transmitted by the antenna scans at constant rate between maximum scan angles of +.theta..sub.0, -.theta..sub.0, .theta. being the angle between the axis of the beam and the normal to the axis of the array. The output of the monitor receiver is sampled at non-uniform intervals of time. The sampling interval does, however, correspond to equal intervals of the arcsine of the scan angle .theta..
The data collected by the monitor receiver is processed by a Fast Fourier Transform (FFT) to provide a recovered illumination function. The recovered illumination function shows the relative amplitude and phase of the energy distributed across the array aperture which created the received signal. Comparison of the recovered illumination function at each sampling point with design values of the illumination function for each element of the array reveals any specific array element at which the amplitude or phase has departed from allowable tolerance.
An amplitude fault usually requires removal of the array from service for repair of the r.f. transmission line, connectors or couplers associated with the faulty array element. Phase faults may be of such nature that correction can be accomplished by simply adding a compensating factor to the phase increment generated for the faulty element by the system beam steering unit. Since the invention identifies the particular element or elements of the array responsible for any degraded performance of the array, the down-time required for repair of the system is much less than in a system having a monitor which warns only of unacceptable performance.
The invention is based upon the accepted theory that the radiation pattern of an antenna can be synthesized from a known aperture illumination function by computing the Fourier transform of the illumination function.
A plurality of receivers placed with equal spacing along a line of length subtending an angle .theta.=arc sin(.lambda./d),
where d is the interelement spacing of the array, would provide n individual signals which may be vectorially summed to provide the Discrete Fourier Transform (DFT) of the illumination function. Then the Inverse Fourier Transform (IFT) of the DFT provides the reconstructed array illumination function.
By sampling the output of a single monitor receiver at non-uniform intervals the invention provides data equivalent that which would be provided by the above described plurality of receivers. Specifically, the time at which the monitor receiver output is sampled is determined by the relationship: ##EQU1## where t.sub.k is the time interval between commencement of the antenna beam scan and the Kth sample;
.lambda. is the wavelength of the radiated energy; PA1 d is the spacing between radiating elements of the array; PA1 N is the number of radiating elements of the array; PA1 t.sub.o =scan start time; and PA1 .theta..sub.s is the beam scan rate in radians/sec.
The samples thus taken by the single receiver are stored in memory in proper order to facilitate processing by a Fast Fourier Transform (FFT) algorithm, the result of which is the reconstructed aperture function of the array. Comparison of the coefficients of the reconstructed aperture function with design values for the corresponding coefficients of the aperture function enables individual identification of radiating elements which are defective in amplitude or phase.